Electrolytic copper foil is produced by passing a stream of electrolyte between an anode of insoluble metal and a rotatable metallic cathode drum mirror-polished on the surface and supplying a potential between the anode and the cathode drum, thereby causing electrodeposition of copper on the cathode drum surface, and, when the electrodeposit has attained a predetermined thickness, peeling the same from the cathode drum. The copper foil thus obtained, called an untreated foil, is thereafter variously surface-treated to be final products.
FIG. 1 illustrates the relative position of a cathode drum and an anode conventionally used for the manufacture of copper foil. In an electrolytic cell (not shown) containing an electrolyte, the cathode drum 1 is installed to be rotatable (clockwise in this case) as partly submerged in the electrolyte. The anode is disposed as divided into, e.g., two anode sheets 3 to cover generally the submerged lower half of the cathode drum 1 in spaced relation with a given clearance from the drum surface. Inside the electrolytic cell, the electrolyte is supplied at 6 o'clock position (of the hour hand, the same applying hereinafter) between the two anode sheets 3. It flows upward along the space between the cathode drum and the anode and overflows the upper edges of the anode for circulation in the cell. A rectifier 5 maintains a given current between the cathode drum and the anode.
As the cathode drum 1 rotates, the electrodeposit of copper from the electrolyte becomes thicker, until it attains a desired thickness around 9 o'clock position, and the resulting untreated foil of the desired thickness is peeled off by suitable peeler means from the drum and is wound up.
In the apparatus for manufacturing electrolytic copper foil, when the operation has continued for a given time period, the anode, among others, is locally worn with use. Consequently, the space between the cathode drum and the anode sheets becomes uneven and the resulting untreated foil becomes uneven in thickness, depending on the characteristics of the apparatus used, till it becomes unmarketable. That is, by the lack of uniformity of the distance between the anode and the cathode drum and the variation of the flow velocity of the electrolyte being supplied etc., the resulting untreated foil undergoes variation in thickness in the direction of the width.
The resulting foil develops variation in thickness in the direction of the length too, in approximately the same pattern per revolution of the cathode drum, due largely to the eccentricity of the rotating cathode drum with periodic variation of the spacing between the anode and the cathode drum.
FIG. 2 is a schematic representation of an illustrative thickness distribution in a half-width section of a copper foil produced. The remaining half-width section not shown has a generally similar variation in thickness widthwise. Thus, the electrolytic copper foil conventionally produced has inevitable variations in thickness in both width and length directions, to varying degrees depending on the manufacturing conditions encountered.
In fact, much difficulties are involved in uniformizing the thickness of an electrolytic copper foil. For example, ANSI/IPC-CF-150E (May, 1981) "Copper Foil of Printed Wiring Applications", which is a standard developed by the Copper Foil Subcommittee of the Raw Materials Committee of the Institute for Interconnecting and Packaging Electronic Circuits, provides: "The area weight or thickness of the copper foil shall be within .+-.10% of the values shown in Table 1 for Type E copper (Electrodeposited copper foil) and .+-.5% for Type W copper (Wrought copper foil)." The provision thus allows for as much as 10% variation in thickness for electrodeposited copper foils in the thickness range from 18 to 498 .mu.m. This shows how difficult it is to control the thickness of electrolytic copper foil so as to uniformize it.
Nevertheless, there has recently been strong demand for electrolytic copper foils of uniform thickness throughout. Copper foils are used chiefly for the fabrication of printed circuit boards, and the tendency toward finer circuits to be formed requires uniform etching of the copper foil and for the purpose a copper foil of uniform thickness is strongly necessitated. For the stabilization of its electric characteristics too, the copper foil must have far less variation in thickness than heretofore.
To make an electrolytic copper foil uniform in thickness throughout the direction of its width, the following steps have hitherto been taken:
(1) Anode milling: With an apparatus for the production of electrolytic copper foil, it has been common that anode after runs for a certain length of time is worn out of use, making the space between itself and the cathode drum uneven. As used herein, the expression "out of use" suggests an abnormal rise of the electrolytic voltage or serious unevenness in thickness of the copper foil produced. In order to avoid this, the anode after service for a given time period is cylindrically reformed on the surface by a special cutting tool.
(2) Partial anode cutting: After anode milling, variation of thickness in the direction of the width of the resulting copper foil is measured. According to the data thus obtained, the anode surface is locally cut off to properly correct the thickness of the copper foil.
Such correcting steps being taken until today restrict the variations of electrolytic copper foil thickness in the direction of its width to the order of about 5% of the target thickness. Little attention has been paid, on the other hand, to the variations of thickness in the direction of the length.
These correction working is a time-consuming, laborious work, necessitating long downtime at regular intervals. However, the precision is nevertheless unsatisfactory. These counter-measures cannot cope with the variations in thickness widthwise from uncertain causes for which the anode is not to blame. In addition, the effect of such a measure, if achieved, would be short-lived.